1. Field of the Invention
The present invention relates to an apparatus for driving a photosensitive belt of an electrophotographic printer, and more particularly, to an apparatus for driving a photosensitive belt of an electrophotographic printer applicable to a laser printer for a computer, a copier or the like.
2. Description of the Related Art
An important general principle of an electrophotographic printer, is that a photosensitive belt which is continuously moved is scanned by an optical signal and thereby a latent image is formed on the photosensitive belt for developer liquid to be applied. For example, in the surface of the photosensitive belt charged to a potential equivalent to 600 V, the potential of the points illuminated by the scanned optical signal is dropped to 150 V. In this way an electrostatic latent image is formed.
FIG. 1 shows a schematic diagram of a general electrophotographic printer. Referring to FIG. 1, a printer 10 includes a photosensitive belt 14 which is moving around a driving roller 11, a tension roller 12, guide rollers 13, as well as a developing device 15 and a transfer roller 17.
The entire width of the surface of the photosensitive belt is charged to a potential equivalent to 600 V. A laser scanning unit 20, installed adjacent to the developing device 15, scans the surface of the photosensitive belt 14 by means of a laser beam. The laser beam is switched so as to selectively illuminate the belt across its width, based on the image to be made. Accordingly, the potential of the illuminated points along the scanned portion of the belt is dropped to 150 V, and an electrostatic latent image is thereby formed.
A plurality of developing devices 15 may be employed according to the number of colors, for example, black, yellow, cyan, magenta, etc. Here, the number of laser scanning units 20 is the same as that of the developing devices 15. Each developing device 15 is provided with corresponding color developer. The developer is applied by a developing roller 15a to the points forming an electrostatic latent image on the surface of the photosensitive belt 14. A backup roller 16a assists the action of the developing roller 15a.
The developer T applied to the photosensitive belt 14 is transferred to printing paper 19 via a transfer roller 17. Here, the printing paper 19 passes between the transfer roller 17 and a pressure roller 18. A backup roller 16b assists the action of the transfer roller 17.
In the electrophotographic printer as described above, a DC (direct current) motor is conventionally employed to move the photosensitive belt 14 at a constant speed. Referring to FIG. 2, a conventional photosensitive belt driving unit comprises a DC motor 23, a tachometer 24, a microprocessor 21 and a driver 22. The DC motor 23 drives the photosensitive belt 14 (FIG. 1) via the driving roller 11 (FIG. 1). The tachometer 24 generates a speed signal proportional to the rotation speed of the DC motor 23. The microprocessor 21 compares the speed signal input from the tachometer 24 with a preset speed signal and outputs a modified speed signal, i.e., a speed control signal to the driver 22. The driver 22 applies to the DC motor 23 driving voltages adjusted according to the speed control signal from the microprocessor 21. Accordingly, since the DC motor 23 rotates at a preset speed, the photosensitive belt 14 is driven at a constant speed.
When the photosensitive belt 14 is simply driven at a constant speed as described above, there is a high possibility of incomplete scans. In other words, the photosensitive belt 14 may move even in a state that an optical signal from the laser scanning unit 20 has not completely scanned a unit scan line of the photosensitive belt 14. Therefore, this becomes a factor which causes deterioration in printing quality of a printer. Further, since the DC motor 23 is precisely driven, a feedback control is required, and the driving circuit and the algorithm of the apparatus are complicated.